Pillared clays Bronsted acid sites

In Fig. 3.58 is shown the integrated intensities of the band at 1540 cm" for pillared beidellite and montmorillonite against the outgassing temperature. Increasing the calcination temperature prior to pyridine adsorption results in a steep drop in the proton content in the case of pillared montmorillonite, while pillared beidellite keeps its acidity. The steep drop of the Bronsted acid sites observed for pillared montmorillonite was attributed to the fact that, upon thermal activation, the protons migrate into the octahedral layer of the clay, where they induce a premature dehydroxylation. Thus, the acidity is mainly of the Lewis type for samples treated at higher temperatures. A similar result was reported also for bentonites pillared with alumina clusters. ... 

Protons are released upon heating which in part balance the negative charge of the host clay layers. A number of review articles have recently appeared which summarize the synthesis and physical properties of metal oxide pillared days derived fix>m the intercalation of polyoxocations of aluminum, zirconium, chromium and many other metals [10-12]. The Lewis acid sites provided by coordinatively unsaturated metal ion sites on the pillar and the Bronsted addity formed upon thermolysis imparts novel chemical catalytic properties [13,14]. Since the pores between pillars often are larger than those foimd in conventional zeolites, there is considerable interest in the use of metal oxide pillared clays for the processing of large organic molecules, espedally petroleum [14-17]. 

The smectite clays exhibit catalytic activity related to the presence of add sites contained at the surface of the clay layer. The role the pillar plays in the catalytic activity of these materials has not been successfiiUy elucidated due to the overriding activity found in the clay itself. The aluminum pillar, for example, produces both Bronsted and Lewis acidity upon thermal treatment ... 

---